The function of the automatic choke is to regulate the air/fuel mixture during engine startup. It produces a higher concentration of fuel ("richer") when the engine is cold, then gradually increasing the concentration of air (making the fuel mixture "leaner") as the engine warms up. The position of the choke butterfly valve (and thus the air/fuel ratio) is controlled by the round device on the upper right side of the carburetor with a wire attached to it. This is the automatic choke.
The way the choke works is this... When the engine is cold the choke closes up so it's ready for a cold start. When you press the throttle the stepped cam on the left side of the carby will rotate to match the closed choke and so will hold the throttle at hi-idle which is needed to keep a cold engine running. When you turn the ignition on, a heating element (the round thing on the right side (right is right of car) at the top of the carby - held by a retaining ring with three screws).
Inside this device is a bi-metallic spring that is electrically heated, hopefully at the same rate at which the engine warms up. As it warms up it slowly opens the butterfly valve in the throat of the carburetor, producing a progressively leaner fuel mixture. At the same time the hi-idle starts to drop back to a normal idle. Once the engine is fully warm, the choke butterfly will be fully open (i.e., standing straight up), producing the proper fuel/air mixture for the fully warmed engine.
On a cold morning before you start the engine, take the air cleaner off and look down the carby throat. Just under the accelerator pump delivery tube is the choke butterfly. pull the throttle arm out gently and the choke should close across the throat (you have to move the throttle arm to set the choke on). If it doesn't close almost right up, it can be adjusted.
The automatic choke is located on the right side of the carburetor at the top. Its held in place by a metal ring with three screws. There is a single wire running to it from the + side of the coil. Inside the automatic choke there is a bi-metallic coil that is warmed by the connection to the coil and unwinds hopefully at the same rate as the engine warms. It is the unwinding of this bimetallic coil that gradually opens the butterfly valve in the throat of the carburetor.
If you want to, you can check that the wire connector on the choke is getting power by using a VOM (Multimeter) to check for 12v with the ignition switch on, or even a 12v light bulb will do.
The automatic choke is adjusted by loosening the three screws in the metal ring that holds it in place. The choke canister can then be rotated underneath the ring. Turning it anti-clockwise (viewed from the right side) increases the amount of choke (a detailed Automatic Choke Adjustment Procedure is given on our main Web site).
Regarding the operation of the automatic choke, Rob has written:
The interesting bit is that this is a 'timed' device. It opens at a set rate as soon as the ignition is turned on (by way of a wire connected to the + side of the coil. This is designed to match the engine warming up, provided you drive off as soon as you start up (VW recommends this) rather than 'warming it up' first; and provided you still have the cooling flaps in the fan shroud (these close off a lot of the cooling air when the engine is cold -- speeds up engine warming).
If you have no cooling flaps, and 'warm it up' by idling for a few minutes, the choke will open up assuming the engine to be warm, when it isn't! So it dies at idle for some time until the engine does get up to running temperature. In essence, the engine runs 'lean' in this condition, which is why you can restart it easily with a few pumps on the throttle (squirts more fuel in). Same story with 'catching' it before it dies, a hard pump on the throttle gives it a squirt of fuel which replaces the missing rich mix it was expecting from the (now open) choke.
Note: If your new spark plugs very black and sooty very quickly, this may be an indication that the choke is not working (won't turn off - the butterfly valve remains closed).
Adjustment of the automatic choke is very important; again, detailed instructions for Automatic Choke Adjustment are given on our main Web page.
Troubleshooting -
First check to make sure that the wire from the coil is connected to the heater element on the choke (high on the right side of the carby).
If it is connected, then it SHOULD be opening the choke as the engine warms up.
A "sticking" choke could be a physical problem with the butterfly itself, or it could be a broken choke element. Both of these are replacable separately. If you have a VOM (Multimeter) you can test it easy enough. Set the scale to about 10 ohms (not critical but a low figure and it must be ohms) and touch the probes to the element connector and the metal body of the carb (pull the wire off the connector before the test). If you get any reading the element is intact and if you get no reading it's broken.
If the carburettor is actually getting old and DOES need replacing, it can be purchased complete. If you want to test it before you decide on replacing the choke element or whole carb, you can wire the choke butterfly open (using the stepped cam and the little hook on it which is connected directly to the choke butterfly), and drive the car around. It will be hard to start without the choke but once warmed up it should drive smoothly - if it doesn't then the whole carb MIGHT be ready for replacement, or it could just need a tuneup in which case you can use the tuneup procedure on our Web site.
If it still runs badly after doing that tuneup then the carb is almost certainly toast.
In summary -
Question -
Not long after purchasing our '72 SB we started to experience cold starting troubles and observed that the upper butterfly in the did not operate. Quick removal of the automatic choke and a reinstall again, everything is fine for a few months. Then the cycle starts again. Any insight as to why this would continue to happen?
Response -
That's an unusual problem - I presume you are saying that the pin which operates the choke inside the choke canister is slipping off all the time. I haven't heard of this happening before, and I don't have any immediate answers for you.
Lets see if I can work through this.
The choke butterfly has a pin which sticks through the choke canister, through a curved slot in a circular plastic "dish" (for want of a better word.)
The choke coil spring sits inside the outer cover (which has the choke wire connector on the outside of the cover). This is held in place by the retaining ring with three screws.
The choke coil spring has a hook on the end of it which wraps round the projecting pin, and pushes the pin around to move the butterfly. Does your choke coil spring have a full U turn in the hook at the end? If it's damaged it could slip off the pin. The pin itself projects nearly, but not quite to the outer rim of the choke canister - maybe 3 mm short of the canister rim. I don't know if it's possible to maybe bend the arm under the plastic dish a little to extend the pin out towards the canister rim.
The plastic "dish" has to sit well down inside the canister - it's rim sits about 4-5mm down inside the rim of the choke canister (a mm or 2 lower than the outer end of the pin). If it's not sitting that far in, it would stop the choke coil spring from sitting inside the canister far enough to make good contact with the pin. In other words, is there something stopping the plastic "dish" from seating properly inside the canister?
New choke spring assemblies (with heating coil behind) are available (through John Connolly at Aircooled.Net for example), if there is something wrong with that part of the choke. You could also ask John (technicalquestions@aircooled.net) your question and he might have a better idea of the cause - he'd certainly be able to tell you if other parts of the choke components are available separately, or if, in the end, replacing the whole carby would get the best result. Roughly $US110 for the H30/31 and roughly $US150 for the larger 34PICT3 (wish we could get them at those prices here in Aus - the H30/31 is about $AU300 and the 34 is over $AU400!
Question -
I just purchased a '69 Bug ... a mechanic is telling me I need a new carb since the choke is stuck; is there anyway to fix or replace the choke without buying an entire new carb? It's a single, automatic. The engine # is AH0395260.
Response -
Since it's an AH engine (a '72 or '73 1600 engine), it should have a 34PICT/3 carburettor. The actual choke butterfly is part of the carburettor and isn't sold separately but the heater element (the "automatic" bit) is replacable separately.
The way the choke works is this... When the engine is cold the choke closes up so it's ready for a cold start. When you press the throttle the stepped cam on the left side of the carby will rotate to match the closed choke and so will hold the throttle at hi-idle which is needed to keep a cold engine running. When you turn the ignition on, a heating element (the round thing on the right side (right is right of car) at the top of the carby - held by a retaining ring with three screws) starts to warm up and slowly opens the choke, and at the same time the hi-idle starts to drop back to a normal idle.
The choke element gets it's power from the black wire connected to the + side of the coil (the same wire also works the idle shut-off solenoid in the left side of the carburettor - this prevents "running on" after you turn the engine off, but isn't part of your current problem - I just mentioned it so you can sort out the wires to make sure the choke is connected right). If you want to, you can check that the wire connector on the choke is getting power by using a VOM (Multimeter) to check for 12v with the ignition switch on, or even a 12v light bulb will do.
So - first check that the wire is connected to the heater element on the choke (high on the right side of the carby).
If it is connected, then it SHOULD be opening the choke as the engine warms up.
The "sticking" could therefore be a physical problem with the butterfly itself, or it could be a broken choke element, and they are replacable separately. If you have a VOM (Multimeter) you can test it easy enough. Set the scale to about 10 ohms (not critical but a low figure and it must be ohms) and touch the probes to the element connector and the metal body of the carb (pull the wire off the connector before the test). If you get any reading the element is intact and if you get no reading it's broken.
Aircooled.Net has them (stock number FSK0013) for about $28 US.
Most other big VW places, e.g., California Import Parts, Ltd., Rocky Mountain Motor Works (now Mid America Motorworks) etc., should have them too.
If the carburettor is actually getting old and DOES need replacing, those same places can supply them complete. If you want to test it before you decide on replacing the choke element or whole carb, you can wire the choke butterfly open (using the stepped cam and the little hook on it which is connected directly to the choke butterfly), and drive the car around. It will be hard to start without the choke but once warmed up it should drive smoothly - if it doesn't then the whole carb MIGHT be ready for replacement, or it could just need a tuneup in which case you can use the procedure on my site http://www.oocities.org/auusiebug1970/34pict3.html .If it still runs badly after doing that tuneup then the carb is almost certainly toast.
In summary -
Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.
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Last revised 4 May 2004.